Set up a double boiler. Add a few inches of water to a medium saucepan and top it with a stainless-steel bowl slightly larger than the circumference of the saucepan. The bowl should create a seal with the bottom saucepan to trap the steam produced by the hot water. Ensure the bottom of the bowl isn’t touching the water – this would cause the chocolate to get too hot. Bring the water to a gentle simmer over medium-low heat. Once the water is hot, turn the heat to the lowest setting.
Melt the chocolate. Add the chocolate to the stainless steel bowl set over the saucepan and melt the chocolate, stirring constantly, until the chocolate is completely smooth.
Blend. Pour the melted chocolate into a high-speed blender, add the bananas, and blend on high until smooth. Be careful not to over-blend the mixture; otherwise, the batter will get too liquidy. Alternatively, you could also mash the bananas with a fork, pour in the melted chocolate, and mix until smooth. You should end up with a thick but pourable batter.
Pour the batter into a springform pan. Transfer the mixture to a lightly greased 6-inch/15-cm springform pan and spread it evenly, smoothing out the top. Tap out any air bubbles.
Freeze. Transfer the chocolate banana cake to the freezer to firm up, for about 2 hours. The more frozen the cake, the easier and cleaner the removal from the springform pan.
Serve. Once set, remove the chocolate banana cake from the springform pan. Cut the cake when it's still semi-frozen to get nice and symmetrical slices, and then store the individual slices in the refrigerator so they're ready to eat.
Store. Leftover chocolate banana cake keeps well in an airtight container in the refrigerator for up to 1 week. For longer-term storage, freeze in an airtight container for up to 1 month.